1. Field of the Invention
The present invention relates to an ultrasonic diagnostic apparatus having a function of measuring data such as intima media thickness (IMT) of a blood vessel based on reception signals obtained by transmitting ultrasonic waves to an object to be inspected and receiving ultrasonic waves from the object. Further, the present invention relates to a data measurement method and a data measurement program to be used for the data measurement.
2. Description of a Related Art
Recent years, intima media thickness (IMT) has attracted attention as an indicator for determination of arteriosclerosis. Referring to FIG. 16, an arterial wall has a three-layer structure including an intima 901, a media 902, and an adventitia 903. IMT refers to a thickness of the intima 901 and the media 902 of them (i.e., a length from a boundary between a vascular lumen 900 and the intima 901 to a boundary between the media 902 and the adventitia 903). From recent research, it has been found that the intima media thickness increases and a plaque is formed as arteriosclerosis progresses. Here, the plaque is a part where a vessel wall is raised inwardly. Tissues of the plaque may be various tissues such as blood clot or fatty or fibrous tissues, and cause carotid artery stenosis, cerebral infarction, cerebral ischemia, and so on.
FIGS. 17 and 18 are schematic diagrams showing part of a carotid artery. As shown in FIGS. 17 and 18, the blood pumped from the heart is introduced into a common carotid artery (CCA) 911, and divided into an external carotid artery (ECA) 912 that connects to an artery of the face and an internal carotid artery (ICA) 913 that connects to an artery of the brain.
The above-explained IMT is measured by ultrasonic examination (carotid artery ultrasonic examination). That is, an ultrasonic probe including an ultrasonic transducer array, in which plural ultrasonic transducers are arranged, is brought into contact with the cervical part of an object to be inspected (a patient) to transmit ultrasonic waves. Here, the reason the IMT is measured in the carotid artery is that the carotid artery is a favorite site of arteriosclerosis. In this regard, the plural ultrasonic transducers are sequentially driven and an ultrasonic beam is formed by synchronization of plural ultrasonic waves, and thereby, the object is electronically scanned. Thus transmitted ultrasonic waves are reflected on the surface of a structure within the object (a boundary between different tissues), and resulting ultrasonic echoes are received by the ultrasonic probe and reception signals are generated. These reception signals are processed in an ultrasonic diagnostic apparatus main body connected to the ultrasonic probe, and thereby, an ultrasonic image is generated. Further, an examiner (an operator such as a doctor) measures the vessel wall by using a vernier caliper or the like in the ultrasonic image generated as described above, and therefore, the IMT is obtained. Furthermore, the level of arteriosclerosis is measured based on the IMT and the vessel status throughout the body including the heart and the brain is estimated based on the result.
However, according to the measurement method, there are problems that the measurement requires long time and the measurement accuracy largely varies depending on the levels of skill of examiners. In order to actively utilize IMT in mass checkup or the like, efficient IMT measurement requiring short time and providing measurement results that vary little depending on examiners is desired.
As a related technology, Hiroyuki TOIDE, “PROPER ULTRASONIC EXAMINATION OF BLOOD VESSELS”, Ultrasonic examination technique, Vol. 31, No. 2 (2006), pp. 80 discloses precautions and points for ultrasonic examination of blood vessels. In carotid artery ultrasonic examination, the vessel diameter, maxIMT shown in FIG. 17, and meanIMT shown in FIG. 18 are measured. The meanIMT is obtained by measuring maxIMT and two IMTs at positions “a” and “c” on both sides at 1 cm from it, and calculating an average value of the three points as follows: {maxIMT+IMT(a)+IMT(c)}/3. According to TOIDE, in IMT measurement, measurement is performed in two positions of the common carotid artery 911 and a region from the vascular bifurcation 914 to the internal carotid artery 913. A plaque 915 is likely formed in a region where the direction of blood flow changes like in the vicinity of the entrance of common carotid artery 911 or the vicinity of vascular bifurcation (BIF) 914.
Further, Japanese Patent Publication JP-B-2889568 (Japanese Patent Application Publication JP-A-11-318896) discloses an intima media thickness measurement apparatus including an ultrasonic unit that outputs data of images imaged by using ultrasonic waves as digital data, a data transmission unit that transmits the digital output of the ultrasonic unit by using optical coupling, and a data analysis unit that calculates the intima media thickness of a blood vessel based on the image data of the blood vessel transmitted by the data transmission unit, and the data analysis unit calculates a reference position based on a moving average value of intensity values of the digital image data and calculates the intima media thickness of the blood vessel based on the local maximum value and the local minimum value of the intensity values within a predetermined pixel range from the reference position toward the vessel wall of the blood vessel.
In JP-B-2889568, the IMT value is automatically calculated by searching for peak values of intensity. However, which points (local maximum points or local minimum points) are used in IMT measurement depends on examiners' criteria and preferences. Further, it is not always to obtain desired results by using the same points at every time because the statuses and conditions of ultrasonic images are not necessarily the same at every time in view of the influence of noise. Furthermore, in JP-B-2889568, the operation in the case where no desired measurement result is obtained (e.g., recurrent computation) is complex, and therefore, efficient examinations can hardly be performed in medical checkups or the like. Moreover, no method of calculating maxIMT or mean IMT is mentioned in JP-B-2889568.
Japanese Patent Application Publication JP-P2005-118314A discloses an ultrasonic diagnostic apparatus including an ultrasonic probe that transmits ultrasonic waves into a body of an object to be inspected and receiving the ultrasonic waves reflected within the body to convert them into high-frequency electric signals, A/D converting means for converting the high-frequency electric signals into high-frequency digital data, high-frequency digital data storing means for storing the high-frequency digital data, image data converting means for converting the high-frequency digital data into digital data for image display, image display means for displaying images based on the data for image display, and data analysis means for acquiring the high-frequency digital data from the high-frequency digital data storing means and performing a predetermined analysis thereon. The data analysis means detects inner walls of intima and adventitia of a blood vessel and calculates intima media thickness.
In JP-P2005-118314A, RF data is stored in the CINE memory and predetermined data processing (e.g., filtering processing for removing noise) is performed on image data that has been converted from the RF data, and IMT is automatically calculated based on the image data after data processing (paragraphs 0022, 0026, 0027). Then, if no desired calculation result is obtained, the parameters for data processing such as filtering processing is reset and calculation for obtaining IMT is performed again (paragraph 0030). However, since the RF data is large in data amount, the cost of the CINE memory becomes high. Further, since resetting of parameters must be done by the examiner, the efficient examinations can hardly be performed in medical checkups or the like and variations in results may be caused depending on examiners.